1. Field of the Invention
The present invention relates to an optical waveguide device such as an optical modulator.
2. Background Arts
In Japanese Patent Publication No. 2004-219600A, it is described to reduce the film thickness of a modulating electrode of an optical modulator so that the propagation loss in the electrode can be reduced and the velocity matching can be attained at the same time. This reference describes an X-plate of lithium niobate single crystal as an optical waveguide substrate and an optical waveguide provided in a gap between signal and ground electrodes. The optical waveguide is provided near the signal electrode in an optical modulating region modulating light by applying a voltage.
According to Japanese Patent Publication No. 2005-331531A, the widths of the gaps of the signal and ground electrodes are changed between voltage applying and feed-through regions. It is thereby possible to prevent ripples in the radio frequency pass characteristics (S21) of the electrodes.
Further, according to Japanese Patent Publication No. 2007-133135A, the radius of curvature of a curved part of a Mach-Zender type optical waveguide is made 30 mm or smaller so as to reduce the radiation loss in the curved part.
Further, according to Japanese Patent Publication No. 2002-169133A, the applicant discloses the method of reducing the product VπL of half value voltage and electrode length by approaching the optical waveguide to the central electrode.
Besides, according to WO2007/058366A1, the applicant discloses a folded-back type optical modulator having a specific structure.
Further, according to Japanese Patent Publication No. 2003-215519A, the applicant discloses the technique of attaining the velocity matching required for the operation at radio frequency by thinning an optical waveguide substrate.